The present invention relates to ultra-wide band optical digital coherent detection.
For beyond 100 GbE technology development, optical superchannel has been one of the attractive options. Optical superchannel employs multi-carrier transmission to scale the channel capacity to 400-Gb/s, 1-Tb/s, or above, while managing the subcarrier spacing effectively so the spectral efficiency for fiber communication can be much improved from conventional DWDM with fixed channel spacing. However, the reduced subcarrier/channel spacing within an optical superchannel poses new challenges to perform optical switching on the subcarriers within the superchannel. Conventional optical wavelength selective switches (WSS) have switching resolution limit typically between 12.5˜50 GHz, therefore cannot handle subcarrier switching in superchannel design where guard bands are typically smaller than 5 GHz.
There are two major categories for subcarrier switching within optical superchannel: inter-transceiver superchannel switching and intra-transceiver superchannel switching. For inter-transceiver switching, each optical subcarrier will be generated and detected using separate transceiver, thus the switching will have to be done at the granularity of each transceiver's operating baud-rate. Since system designer will want to lower the cost per transmitted bit by increasing of transceiver data rate and reducing the number of RF and optical components (i.e. drivers, modulators, and photodiodes), only coarse granularity can be achieved for inter-transceiver switching. Therefore it has lower flexibility for network grooming/switching is low, and more suited for submarine/long-haul where traffic patterns are stable. The main stream method to achieve inter-transceiver switching is to use flexible-band WSS on Nyquist-shaped subcarriers, where the minimal spacing or guard-band is limited by the spectral resolution of the WSS. Very high spectral resolution WSS has been demonstrated for reduction of achievable subcarrier guard-band at the expense of reducing the WSS bandwidth, which will significantly increase the cost and complexity of ROADM design. There are also several proposals for subcarrier switching for all-optical (AO) OFDM superchannel. AO-OFDM has not been adopted by the industry due to its higher cost and inferior performance compare to Nyquist signaling. The switching for AO-OFDM is also not very practical due to the added complexity of optical carrier phase recovery via optical phase-lock loop.
For intra-transceiver switching, each superchannel is consisted of one or multiple optical subcarriers containing finer electrical subcarrier/subbands, thus grooming/switching can be done at portions of each transceiver's data-rate. The finer granularity will promote greater network flexibility and is suitable for adaptation to dynamic traffic patterns and spectrum utilization, as the case in metro/access networks. Other than enhanced flexibility, there are also potential benefit of improved transmission performance as lower fiber nonlinearity distortion can be achieved at certain sub-band granularity which is less than the standard transceiver baud-rate of 32 GHz. Previous demonstrations typically focus on digital sub-banding scheme which relies on “digital transmitters” to convert the digitally generated and multiplexed sub-bands to baseband signal through high-speed DAC. There are two challenges for adopting digital sub-banding scheme: First, compare to the single carrier (one sub-band), which is the standard industry DSP platform, it would require huge investment from the chip designer to develop new DSP to achieve digital sub-banding for the added benefit. Secondly, if optical switching is applied, the sub-band granularity will also be limited by the WSS resolution as discussed earlier. Switching can also be done via OEO technique, where an additional transceiver is used to first recover all the sub-bands and then drop/switch the sub-bands accordingly. This will significantly increase the system cost for ROADM design.